How to Properly Import from Blender to Unreal 4?
This is just a warning, not an error. It's usually the case when UVs aren't unwrapped well; either there are overlapping UVs, or some UV's are backward. It will still work, and I haven't noticed any problems with shading or texturing on the models I've imported into Unreal.
There is apparently a way to get Unreal to correct some of these problems for you.
From the Unreal forums:

Double click onto your static mesh in the UE4 -&gt; Select the recompute tangents/normals and click save